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Alexander S. Aruin and Gil L. Almeida

The hypothesis that persons with Down syndrome are likely to trade mechanical efficacy of motor patterns for safety was tested in a study of postural adjustments associated with fast voluntary movements. Control subjects and subjects with Down syndrome performed bilateral shoulder flexion and extension movements “as fast as possible” while standing on a force platform. Anticipatory changes in the background activity of postural muscles were seen in both groups of subjects. Subjects with Down syndrome demonstrated simultaneous EMG bursts in postural “agonist-antagonist” pairs. Subjects with Down syndrome also demonstrated patterns of deviations in the major postural joints, suggesting that they were using a different strategy which may be a correlate of “clumsiness.” It was concluded that practice in conditions of changing components of a motor task is the most promising way to encourage the central nervous system of persons with Down syndrome to search for alternative strategies and improve these individuals' ability to solve everyday motor problems.

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Slobodan Jaric, Charli Tortoza, Ismael F.C. Fatarelli, and Gil L. Almeida

A number of studies have analyzed various indices of the final position variability in order to provide insight into different levels of neuromotor processing during reaching movements. Yet the possible effects of movement kinematics on variability have often been neglected. The present study was designed to test the effects of movement direction and curvature on the pattern of movement variable errors. Subjects performed series of reaching movements over the same distance and into the same target. However, due either to changes in starting position or to applied obstacles, the movements were performed in different directions or along the trajectories of different curvatures. The pattern of movement variable errors was assessed by means of the principal component analysis applied on the 2-D scatter of movement final positions. The orientation of these ellipses demonstrated changes associated with changes in both movement direction and curvature. However, neither movement direction nor movement curvature affected movement variable errors assessed by area of the ellipses. Therefore it was concluded that the end-point variability depends partly, but not exclusively, on movement kinematics.